1. Field of the Invention
The general field of the invention is the fabrication of carbon-fiber-reinforced carbon-matrix composites, commonly referred to as carbon-carbon or C/C composites. The invention applies particularly to carbon-carbon composites fabricated with pitch-based matrices.
2. Description of the Prior Art
Although carbon-carbon composites now find a variety of applications, ranging, for example, from prosthetic implants to components for braking systems and heat exchangers, the major impetus for their further development continues to come from space and defense needs, where the high costs and lengthy processing times of conventional fabrication methods can be justified by unique capabilities of thermal and mechanical performance in the final composite product.
Two types of carbon-carbon composites are widely used: 3D composites that are essentially carbon bodies with some carbon fiber reinforcement effective in all directions, and 2D composites that are usually comprised of one or more parallel plies of fabric woven from carbon fiber. The 3D composites usually take the form of compact bodies that can be used to make nose cones or rocket nozzles for space vehicles. The 2D composites usually take the form of thin- walled tubes, plates, or other shell-like shapes that are particularly useful for structures where lightness, stiffness, and strength are primary considerations.
Three general methods are used in the current art of fabricating carbon-carbon composites. The carbon matrix may be deposited within a carbon fiber preform by: (1) chemical vapor infiltration (CVI) from hydrocarbon gases, such as methane or natural gas; (2) liquid impregnation with thermosetting resins that harden before carbonization reactions begin; or (3) impregnation with liquid pitches derived from petroleum or coal tar. Repetitive cycles of impregnation and carbonization are usually required to attain useful density levels. In fabrication practice, the method of impregnation may be varied from cycle to cycle.
As presently practiced, all processes are slow and expensive, and fail to realize fully the strength of the reinforcing fibers. Thus a situation has come to exist in which carbon-carbon composites find use in critical applications where no other material could serve, but serious constraints of cost, processing time, and reliability in fabrication limit their development for other applications where such properties as stiffness or refractoriness could provide substantial advantages over other structural materials.
In applying the process of chemical vapor infiltration, it is essential that an appreciable fraction of the reactive gas species diffuse into the full depth of the porous body before carbonization. To maintain such "throwing power," the process must be restricted to pressure and temperature conditions that make the CVI process inherently slow, expensive, and suitable primarily for thin-walled 2D composites. In practice, "bottleneck" pores limit the attainable density levels, and the formation of external crusts of pyrolytic carbon requires the process to be interrupted for cleaning between cycles. Nevertheless CVI processing is a significant competitor in the prior art of carbon/carbon fabrication and may find an important role in combination with liquid impregnation processes as a final infiltration step to control the degree of fiber-matrix bonding.
Thermosetting resins have been well-exploited for carbon-carbon fabrication because an extensive technology base (in fiber-reinforced plastics) exists for these matrices that can be fixed in place prior to carbonization. However this thermosetting characteristic also leads to the formation of glassy carbons that are inherently brittle, low in density, and difficult to graphitize. The commonly used impregnants are phenolic resins that carbonize with only modest yield; although higher-yield resins are being explored, they tend to be costly and lacking in technology base. The linear shrinkage in carbonization is typically about 20% and can cause severe fiber damage if benign patterns of shrinkage fracture do not form in the matrix. These difficulties require lengthy steps of curing and carbonization and a number of repetitive cycles to reach density levels of 1.65 g/ml.
Pitch-based processing methods emerged from the technology base established by the manufacture of artificial graphites. These processes seek to realize the advantages of high carbon yield, excellent graphitizability, and lower cost that are characteristic of pitches derived from coal tar or petroleum. Such pitches are comprised primarily of polynuclear aromatic molecules. Upon pyrolysis, reactions of aromatic polymerization carry the pitch through a liquid crystalline state, known as the carbonaceous mesophase, in which graphitizability is established by parallel alignment of the large flat aromatic molecules. Graphitizable precursors produce cokes of higher density, and graphitic matrices are less brittle than glassy carbon matrices.
These advantages have led to a substantial carbon-carbon fabrication technology based on pitch impregnants. However the carbonaceous mesophase is a viscous reactive liquid in the practical ranges of processing variables, and the gaseous reaction products cause bloating effects that can seriously reduce densification efficiency. That mesophase pitches bloat seriously upon carbonization, even under substantial applied pressures, is known from studies of petroleum coking in which foaming has been found to commence when the coke feedstock transforms to bulk mesophase, and the microstructure of the coke has been shown to depend on the amount of deformation by bubble percolation. Experience with pitch-based composite processing shows that it is difficult to achieve composite densities greater than 1.6 g/ml as long as room-pressure carbonization methods are used. Thus although the mechanisms defeating the attainment of high composite densities are different than in the case of processing with thermosetting resins, the ultimate practical density levels are comparable for the two conventional approaches to room-pressure densification by liquid impregnation.
The prior art solution to the difficulties of densification with pitch impregnants is to use an autoclave to apply high pressure during the pyrolysis process until the matrix hardens to coke. This approach is attractive not only to reduce the volume of bubble porosity within the matrix, but also because high-pressure pyrolysis offers a potential increase in carbon yield. In practice the autoclave systems are run at pressures of the order of 15 kpsi to temperatures of about 600.degree. C. This necessarily involves a substantial capital investment in autoclaves, control equipment, and safety facilities. Furthermore high-pressure processing is limited by practical autoclave sizes.
However detailed analyses of the efficiency of carbon pickup within 3D preforms for each cycle of impregnation and carbonization reveal that the gains in densification by pressure pyrolysis are less than might have been expected. The difficulty in improving the efficiency appears to lie in the fact that the gases emitted during the final stages of mesophase formation and hardening are principally methane and hydrogen, both of which are non-condensable under the pyrolysis conditions. Although pressure applied during pyrolysis reduces the volume of gas evolved in the critical range of mesophase hardening, the volume is still sufficient to expel appreciable amounts of matrix from the fiber preform, and thus to reduce the cycle efficiency.
In fact some bubble porosity seems to be inevitable in any pitch-based process as long as thermal methods are used to harden the mesophase matrix, because the non-condensable gases are essential products of the polymerization reactions that effect the hardening. In this sense high-pressure pyrolysis is an incomplete and less than satisfactory solution to the problem of attaining efficiency in pitch-based densification.
3. Objects of the Invention
The immediate and direct object of the present invention is to provide a means of fixing pitch-based matrices in place within fiber preforms so that carbonization processes may be applied in composite fabrication without the risk of matrix expulsion from the preform by pyrolysis generated gases. It is also the object of this invention to stabilize the matrix micro- structure, as established by the impregnation process, to survive carbonization without modification. It is a further object to enhance the carbon yield of the pitch matrix in the carbonization process.
Another immediate and direct object of the present invention is to eliminate the need for costly high-pressure pyrolysis and carbonization facilities in the pitch-based processing of carbon-carbon composites. It is also the object to reduce fabrication costs by using cheap materials and reactants, for example, petroleum or coal-tar pitch as matrix precursor, and using air to oxidize the matrix.
These specific objects contribute to several general objectives. By eliminating the need for high-pressure facilities, pitch-based fabrication can be extended to 2D composite structures that are too large to be economically treated in high-pressure autoclave systems. By avoiding matrix expulsion and enhancing carbon yield, the number of process cycles required to attain desired density levels in both 2D and 3D composites can be reduced, with attendant economies in time and cost.
By providing a means of retaining pitch-based matrices in place during carbonization, the process of oxidation stabilization can be combined with any process for putting a pitch or mesophase pitch matrix into place. In this way the art of pitch-based composite fabrication can be enlarged to use a broader range of impregnation or injection processes.